Liquid membrane

ABSTRACT

A liquid membrane is disclosed, which comprises a solvent, an active species capable of facilitated transport of a specific gas, and a support for maintaining a liquid body which has a mixture of said solvent and active species dissolved therein, said support being a porous polytetrafluoroethylene film one surface of which is hydrophobic and the other of which is hydrophilic.

BACKGROUND OF THE INVENTION

The present invention relates to a liquid membrane used in facilitatedtransport of a specific gas. More specifically, the invention relates toa liquid membrane using a porous polytetrafluoroethylene support onesurface of which is first roughened by a physical means, wherein thesupport has a polymerized layer of a nitrogen-containing compoundchemically deposited thereon so as to provide an increased affinity forliquids.

Membrane technology using ethyl cellulose, acetyl cellulose orsilicon-carbonate copolymer membranes to separate a specific componentgas from a gaseous mixture containing the same has been the subject ofextensive research. These membranes are made of solid materials and arehence referred to as solid membranes. In order to separate the specificgas component in an economical and efficient manner, two requirementsmust be met: (1) a membrane material that permits the selective passageof the gas component to be separated must be selected; and (2) themembrane must be made as thin as possible. Most of the materials knowntoday for use in solid membranes have low gas selectivities and thosematerials which have relatively high selectivities are only capable ofvery low rates of gas permeation.

Facilitated tranport of a particular gas can be realized by using aliquid membrane of a material having a high degree of affinity for thegas to be separated. As shown in U.S. Pat. Nos. 3,865,890, 3,951,621,4,015,955 and 4,060,566, ethylene can be selectively concentrated from amixture of methane, ethane and ethylene using a nylon-6.6 membraneimpregnated with an aqueous solution of AgNO₃. The membrane disclosed inthese patents is rendered hydrophilic by incorporation of a hydrophilicpolymer such as polyvinyl alcohol, but the life of the membrane is notvery long since water used as the solvent for the aqueous solution of Agions will unavoidably evaporate during the use of the membrane.

U.S. Pat. Nos. 3,396,510, 3,819,806 and 4,119,408 show that acidic gascomponents such as CO₂, H₂ S and SO₂ can be selectively permeatedthrough a polyethersulfone membrane impregnated with an aqueous solutionof K₂ CO₃. But this membrane has the same problem as in the case ofethylene separation discussed above.

European Patent Application No. 98731/1984 shows that oxygen can beselectively separated from the air using a nylon-6.6 membraneimpregnated with a transition metal complex of a Schiff base dissolvedin a solvent such as lactone or amide. In the Example therein, anylon-6.6 membrane with a thickness of 130 μm was used and this suggeststhe presence of a liquid membrane that was at least 130 μm thick.

This membrane system is substantially free from the problem of solventevaporation because the liquid barrier is made not of water, but oforganic solvents such as lactone and amide. However, the life of thesystem is short because the transition metal complex of a Schiff base isirreversibly oxidized during operation. In additon, the liquid barrierthat is impregnated into the nylon-6.6 membrane cannot be made thinnerthan 130 μm, preferably as thin as a few micrometers.

SUMMARY OF THE INVENTION

One object, therefore, of the present invention is to provide a liquidbarrier in a liquid membrane that is not thicker than a few micrometersand which yet is capable of enhanced transport of a specific gascomponent.

Another object of the invention is to provide a membrane support onesurface of which does not dissolve in solvents but has a suitable amountof affinity therefore. In order to make a liquid barrier not in the formof an aqueous solution but in the form of an organic solution with lowvolatility solvents such as lactone, dimethylformamide andN-methylpyrrolidone are used. These solvents will dissolve known poroussupport materials such as polysulfones and polyamides, so it isnecessary to use a support material that will not dissolve in theseorganic solvents.

DETAILED DESCRIPTION OF THE INVENTION

The liquid membrane in accordance with the invention is composed of asolvent, an active species for performing facilitated transport of aspecific gas, and a support. The support is made of porouspolytetrafluoroethylene (hereinafter abbreviated as PTFE) and ischaracterized by having one hydrophilic surface in order to make contactwith a mixed solution of the solvent and active species and to retainthis mixed solution in the form of a thin film. In order to provide suchhydrophilic surface, the support must be given both a physical treatmentfor roughening one surface and a chemical treatment for depositing alayer of a non-PTFE compound on that surface. If only one of these twotreatments is applied, a liquid barrier that will maintain a uniformthickness cannot be obtained, or even if this is possible, the resultingmembrane has a very short life.

The PTFE used as the support material is insoluble in almost allsolvents known today, so it can be used in supports for liquid barriersemploying solvents of high polarity such as dimethylformamide(hereinafter DMF) and N-methylpyrrolidone (hereinafter NMP). However,because of high polarity, DMF and NMP exhibit such a great surfacetension that they are incapable of wetting the surface of PTFEsatisfactorily. As a result, the porous PTFE membrane cannot be put toservice in any manner other than where a pool of a mixed solution ofsolvent and active species is placed on the membrane, thus producing anundesirably thick liquid barrier.

It therefore becomes important for the purposes of the present inventionto render one surface of the porous PTFE hydrophilic so that it iswettable by polar solvents. The intensity of this hydrophilic treatmentdetermines the thickness in which the liquid barrier made of the solventand active species can remain stable.

The physical treatments for providing a roughened surface includegraining with a metal brush, etc., heat treatment in which only thesurface layer of PTFE is thermally decomposed, laser treatment using thelaser energy as a heat source, low-temperature plasma etching using anon-polymerizable gas, and sputter etching using Rf energy, which aredescribed in U.S. Pat. Nos. 4,297,187, and 4,311,828. By selecting aproper method from among these methods, a surface roughness in the rangeof 1 to 100 μm preferably 2 to 20 μm, can be obtained. If a very finetexture with a roughness of about several micrometers is desired, lasersor Rf energy is preferably used.

U.S. Pat. Nos. 3,664,915, 3,953,566, 4,082,893, and 4,248,942 provideporous PTFE by stretching and expanding operations. The structuresproduced by such techniques consist of fibers and nodes interconnectedby the fibers. Such structures are preferred starting materials sincethey provide for a great latitude in selection of porosity and poresize. If the porous PTFE is subjected to sputtering or plasma etching,the fibers in the treated surface are cut and subsequently decomposed toprovide a texture structure consisting essentially of nodes. If, on theother hand, only the surface layer of the porous PTFE is thermallydecomposed by scanning with a CO₂ laser beam, not only the fibers butalso a part of the nodes is volatilized.

The choice of a suitable method of physical treatment depends on therequired thickness of the liquid barrier.

The physical treatment for roughening one surface of the porous PTFE isfollowed by a chemical treatment. The surface subjected to roughening bya physical treatment has a reduced angle of contact with the highlypolar DMF or NMP and its affinity for these solvents appears to haveincreased in comparison with the untreated surface. In fact, however,the roughened surface of PTFE causes little effects on its inherentnature and the increase in its affinity for polar solvents isnegligible. In order to achieve an increased affinity for such solvent,the surface of the PTFE must be covered with a thin layer of a compoundhaving a chemical structure similar to that of the polar solvents. Thisobject is insufficiently achieved by merely performing "tetraetching",i.e., withdrawal of fluorine atoms using sodiumnaphthalene complex orother suitable etchants. The preferred chemical treatment is onedepending on plasma polymerization for depositing a compound similar tothe solvent component of the final liquid barrier. As described in U.S.Pat. Nos. 3,657,113, and 4,199,448, the plasma polymerization isrealized by introducing a polymerizable gas into a bell jar or a tubularreactor while a glow discharge is maintained by application of radiofrequencies, microwaves or d.c. current. Polymerizable gases that can beused with advantage are nitrogen-containing cyclic compounds thatinclude pyridines such as 4-vinylpyridine, 2-vinylpyridine,4-ethylpyridine and 5-vinyl-2-methylpyridine; pyrrolidone derivativessuch as N-methylpyrrolidone and N-vinylpyrrolidone; amines such as4-methylbenzylamine and N-butylamine; and pyridine derivatives such aspicoline and lutidine. Oxygen-containing compounds could be used aspolymerizable gases but they are by no means advantageous over thenitrogen-containing compounds from the viewpoint of deposition rates ofplasma polymers.

If nitrogen-containing compounds, preferably cyclic compounds, are usedas polymerizable gases, films of plasma polymer are produced that areabout 1 μm thick and which are composed of a highly cross-linkedstructure. Because of the cross-linked structure, such polymer filmswill not dissolve in the highly polar solvents which are used to make upa liquid barrier, but they can be swelled by such solvents. This willprovide a great advantage for the purpose of maintaining a liquidbarrier in the form of a very thin film of a thickness of about severalmicrometers. The nitrogen-containing compounds listed above will alsoact as "axial bases" in the sense of the term used in European PatentApplication No. 98731/1984. It would of course be possible to form filmsof plasma polymer even thinner than 0.1 μm or thicker than 10 μm bychanging the polymerization conditions. However, polymer films thinnerthan 0.1 μm have a smaller ability to maintain a liquid barrier stablyand it becomes difficult to have a uniform and sound liquid barrierspread over a large surface area. If polymer film is thicker than 10 μm,cracks will develop because of the internal stress that has occurred inthe film during plasma polymerization. Even in the absence of anycracking, the film is unstable and may sometimes separate from thesubstrate.

Under these circumstance, the particularly preferred thickness of theplasma polymer film may range from 0.3 μm to 3 μm. Polymer films havingthis thickness range are capable of maintaining a liquid barrier withthe thickness ranging from 0.1 μm to 6 μm.

The other features of the liquid membrane in accordance with the presentinvention may essentially be the same as described in European PatentApplication No. 98731/1984.

The solvents of high polarity include lactams, sulfoxides and amides,and preferably, dimethyl sulfoxide, NMP, propylene carbonate, DMF andgamma-butyrolactam are used. These solvents may containnitrogen-containing compounds such as polyethyleneimine andtetra-ethylenepentamine, or pyridine and pyrrolidone derivatives used aspolymerizable gases in plasma polymerization.

Examples of the active species capable of enhanced transport of oxygenor carbon monoxide gas are complex compounds of transition metals withSchiff bases prepared by dehydrative condensation from ethylenediamineand aldehyde compounds. Illustrative Schiff bases areN,N-bis(salicylidene)ethylenediamine andbis(2-amino-1-benzaldehyde)ethylenediamine. Exemplary transition metalsare divalent elements such as Co, Cu and Fe. These active species aredissolved in polar solvents in amounts ranging from 10⁻⁵ to 10⁻³ mole,preferably 10⁻⁴ to 10⁻³ mole, per unit weight of the liquid barrier.Higher concentrations of the active species will provide increasedinitial gas selectivities, but their characteristics will vary with timeas a result of dimerization or other side reactions. If theconcentration of the active species is less than 10⁻⁵ mole the intendedeffect of the active species is difficult to obtain and only low gasselectivities will result. Therefore, the preferred concentration of theactive species is in the range of 10⁻⁵ to 10⁻³ mole per g of the liquidbarrier.

The process of producing a large-scale module from the liquid membraneof the present invention will start with the shaping of a support thathas been provided with a hydrophilic surface but on which no liquidbarrier is maintained. In order to provide a large surface area, asupport in the form of a bundle of tubes or hollow fibers is packed in amolding container and both ends of the bundle are sealed. The sealantmay be an epoxy resin but silicone rubbers having a greater adhesivestrength are preferred. After the sealant has solidified, part of thesealed portion is cut open so as to provide a separation module havingrespective channels for the feed gas, permeate gas and reject gas.

After forming such module, a mixed solution of solvent and activespecies is introduced in excess amount into the module at the feed gasinlet and the module is pressurized at 1 to 2 kg/cm² with the reject gasoutlet closed. But this pressurization step, the mixed solution isimpregnated into the entire part of the hydrophilic surface of thesupport. Desirably, complete impregnation of the mixed solution isensured by shaking the whole part of the module. Uniform impregnation ofthe mixed solution can be realized by monitoring the flow rate of gascoming from the permeate gas outlet because as the impregnationproceeds, a decreasing amount of gas will come out of the module untilthe point is reached where a minimum flow rate occurs, which indicatesthat the mixed solution has been impregnated uniformly in every part ofthe hydrophilic surface of the support.

The following Examples are provided for further illustration of theclaimed liquid membrane but are not to be construed as limiting theinvention.

EXAMPLE 1

A reactor of the bell jar type having a sheet of Fuoropore FP-010(porous PTFE membrane manufactured by Sumitomo Electric Industries, Ltd.having an average pore size of 0.1 μm) placed on an electrode wasevacuated to a pressure of 0.01 Torr. Glow discharge was conducted witha radio wave (13.56 MHz) applied at a power of 60 watts.

The reactor was supplied with 4-vinylpyridine until the pressure in thereactor was increased to 0.2 Torr. Plasma polymerization was performedfor 30 minutes at a power of 30 watts. The thickness of the polymerdeposit as estimated from the increase in weight was 0.5 μm. The plasmapolymer coat on one side was dipped in a dimethylformamide bath. When itwas recovered from the solvent bath, the treated surface was found tohave a uniform layer of the solvent but only drops of solvent adhered tothe untreated surface, The drops of solvent were wiped off and thethickness of the weight of the support resulting from the deposition ofthat layer was approximately 3 μm.

EXAMPLE 2

Hollow PTFE fibers (outside diameter: 1.1 mm, inside diameter: 0.6 mm,porosity, 35%, average pore size, 0.1 μm) were used as a startingmaterial.

The hollow fibers were prepared by first extruding PTFE tubes by thepaste method (U.S. Pat. No. 4,225,547), then stretching the tubeslongitudinally at a stretch ratio of 2, and finally sintering thestretched tubes at temperatures not lower than 327° C. (U.S. Pat. No.4,082,893). Only the surface of the outermost layer of each tube wasroughened by passing it at a linear speed of 8 m/min, through a furnacewherein flames issued uniformly in the radial direction. Observationwith a scanning electron microscope showed that the scorched surface hada roughness of about 10 to 30 μm.

A plasma polymerization system was supplied with N-vinylpyrrolidone anda plasma polymer was deposited on the outer surface of each of thehollow PTFE fibers under the same conditions as used in Example 1. Thedeposit of the polymer coat was estimated to have a thickness of 0.35μm.

The fibers were dipped in a dimethyl sulfoxide bath. A uniform layer ofthe solvent was formed on the entire periphery of each fiber, and thethickness of the layer was estimated to be about 8 μm by measuring theincrease in the weight of the fiber.

The interior of each of the fibers was pressurized by introducing air ata pressure of 2 kg/cm² with the other end of the fiber closed. Thedimethyl sulfoxide layer remained strongly adherent on the outer surfaceof each fiber, causing no foam in the liquid barrier.

EXAMPLE 3

Hollow PTFE fibers having an outside diameter of 1.0 mm, an insidediameter of 0.4 mm, a porosity of 30% and an average pore size of 0.05μm were prepared as a starting material by modifying the paste extrusionand reducing longitudinal stretching ratio.

The surface of the outermost layer of each of the hollow fibers wasthermally decomposed by scanning with a CO₂ laser (100 watts) beamobtained by focusing with a lens system consisting of a condenser,reflector and a conical mirror. By this physical treatment, not onlywere the fiber portions cut but also the nodes forming the porousstructure were volatilized.

The so treated hollow PTFE fibers were set in a plasma treatment unitwith a tubular reactor so that they could be transported through theunit. The surface of each fiber was treated by an oxygen gas plasma at40 watts, producing a fine texture (0.5 μm roughness) on the surface ofthe outermost layer of each fiber.

The reactor was supplied with 4-vinylpyridine as a polymerizable gas andplasma polymerization was performed at 30 watts on the fibers running at1 m/min. Assuming a uniform deposition was effected, the depositedthickness of plasma polymer was calculated to be 0.3 μm.

A bundle of 3,000 PTFE fibers thus treated was packed in a cylindricalcontainer so that it would have an effective length of 30 cm, leaving a5-cm portion at each end for subsequent sealing. A silicone rubber ofthe addition reactive type was applied to both ends of the bundle andcured to cross-link. One sealed end of the bundle was cut open toprovide an outlet for permeate gas.

A solution having 10⁻⁴ mole/g of N,N-bis(2-aminobenzyl) ethylenediaminedissolved in dimethyl sulfoxide was injected into the cylindricalcontainer at the feed gas inlet and the container was pressurized for 10minutes with a nitrogen gas supplied at 1.5 kg/cm². In the meantime, thecylinder was rotated to ensure that the solution could be impregnatedinto the entire surface of the hollow fibers. Excess solution waswithdrawn from the container, and immediately thereafter, the containerwas placed in a refrigerator for storage at 5° C. or below.

The module was taken out of the refrigerator and air at one atmospherewas pumped into the module at the feed inlet while the outlet forpermeate gas was held at 30 mmHg. On the product side, an oxygen-richair (66% O₂) was obtained with a calculated O₂ permeation rate of4.2×10⁻⁵ cm³ /cm² /sec cmHg.

While the invention has been described in detail and with reference tospecific examples thereof, it will be apparent to one skilled in the artthat various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A liquid membrane comprising a solvent, an activespecies capable of enhanced transport of a specific gas, and a supportfor maintaining a liquid body which has a mixture of said solvent andactive species dissolved therein, wherein said support is a porouspolytetrafluoroethylene film, one surface of which is hydrophobic andthe other surface of which is hydrophilic.
 2. The liquid membrane asclaimed in claim 1, wherein said porous polytetrafluoroethylene iscomposed of fibers and nodes interconnected with said fibers, whereinthe hydrophilic surface of said polytetrafluoroethylene film is formedby both physical and chemical treatments.
 3. The liquid membraneaccording to claim 2, wherein the physical treatment used to produce thehydrophilic surface is surface roughening.
 4. The liquid membrane asclaimed in claim 3, wherein said physical treatment is selected from thegroup consisting of graining with a metal brush, heat treatment in whichonly the surface layer of PTFE is thermally decomposed, laser treatmentusing laser energy as a heat source, low-temperature plasma etchingusing a nonpolymerizable gas and sputter etching using Rf energy.
 5. Theliquid membrane as claimed in claim 3, wherein the resulting surfaceroughness is in the range of 1 to 100 μm.
 6. The liquid membrane asclaimed in claim 2, wherein the chemical treatment used to produce thehydrophilic surface consists of deposition of a plasma polymerized coatof a nitrogen-containing compound.
 7. The liquid membrane as claimed inclaim 6, wherein the nitrogen-containing compound is at least one memberselected from the group consisting of vinylpyridine, 4-ethylpyridine,N-vinylpyrrolidone, N-methylpyrrolidone, 4-picoline, and 3,5-lutidine.8. The liquid membrane as claimed in claim 6, wherein the plasmapolymerized coat has a thickness of 0.1 μm to 10 μm.
 9. The liquidmembrane as claimed in claim 8, wherein the plasma polymerized coat hasa thickness of 0.3 μm to 3 μm.
 10. The liquid membrane as claimed inclaim 1, wherein the solvent is one or more members selected from thegroup consisting of gamma-butyrolactone, N-methylpyrolidone,4-dimethylaminopyridine and 4-aminopyridine, and the active species is acomplex compound of a Schiff base and a transition metal.
 11. The liquidmembrane as claimed in claim 10, wherein said Schiff base is selectedfrom the group consisting of N,N-bis(salicylidene)ethylenediamine andbis(2-amino-1-benzaldehyde)ethylenediamine and said transition metal isselected from the group consisting of cobalt, copper and iron.